The present invention relates generally to an electronic compass and, more particularly, to a system for and a method of calibrating the compass and displaying a designation of direction heading on the display of the compass.
Although its cause is still the subject of dispute, it is well understood among scientists that planet Earth has its own natural magnetic field. As was discovered early in world history, the Earth""s magnetic field can be used to indicate directional heading. A device that senses the Earth""s magnetic field and aligns a ferromagnetic pointer with the flux lines of the Earth""s magnetic field is generally referred to as a compass. This alignment allows for an identification of directional heading. Compasses are classified within a family of instruments referred to as magnetometers, which function to detect and measure the magnitude and/or direction of magnetic fields.
Modern mechanical compasses typically include a needle mounted for rotational movement that aligns itself with the magnetic flux lines of Earth""s magnetic field. As is well understood, by convention, the magnetic flux lines are said to terminate at the Earth""s magnetic north pole. Typically, during operation, a compass needle points towards the Earth""s magnetic north pole, which is located in Canada.
The ability to identify direction, which is provided for by a compass, has proven to be invaluable for navigators and other travelers. Accordingly, since the development of the very first compasses, navigators have used such devices to sense and identify the directional heading traversed by their vehicles. Today, besides being a favorite among campers, compasses are found on board a variety of vehicles, including airplanes, ships, boats and automobiles. Compasses are increasingly used in position and navigation systems for vehicles.
One drawback of modern mechanical compasses is their inability to be calibrated to eliminate errors due to the magnetic field signature of nearby ferromagnetic material having relatively high permeability. It is understood by those skilled in the art that vehicles ordinarily have their own unique and distinct magnetic field signatures. In other words, the material from which a vehicle is made produces magnetic interference, which combines with the Earth""s magnetic field. As will be appreciated, a compass installed in the vehicle will read the combined magnetic field (i.e., the Earth""s magnetic field and the magnetic field corresponding to the vehicle""s magnetic signature), and will in all likelihood generate inaccurate results as to the vehicle heading. In some cases, the effect of the vehicle magnetic field on the compass reading substantially overrides the effect of the Earth""s magnetic field on the compass reading. When that occurs, the compass is rendered useless as a directional heading identification instrument.
With the advent of electronic technology in modem consumer products, electronic compasses have been developed. Electronic compasses provide substantial benefits over mechanical compasses for several reasons. One such reason is that electronic compasses can be more readily calibrated so that they eliminate the aforementioned vehicle signature magnetic field effect.
Electronic compasses produce electrical signals indicative of directional heading, based upon a measurement of the magnetic field intensity of the Earth""s magnetic field relative to the orientation of the compass. Conventionally, electronic compasses include two distinct data channels defined by two orthogonally disposed magnetic field sensors aligned in the same plane. As a practical matter, the sensors are mounted on the same printed circuit board (PCB) positioned inside the housing of the electronic compass. As is well known in the art, the sensors generate electrical signals representative of the sensed magnetic field intensity of the Earth""s magnetic field.
For purposes of representation and to facilitate an understanding of those signals, the respective magnitudes of the output electrical signals for each sensor may be represented by the magnitudes of component vectors on respective axes of a reference Cartesian coordinate system. In particular, the magnitude of the electrical signal for a first magnetic sensor may be represented by the magnitude of a first component vector extending in the same direction as the abscissa axis of a reference Cartesian coordinate system. On the other hand, the magnitude of the electrical signal for the second magnetic sensor may be represented by the magnitude of a second component vector extending in the same direction as the ordinate axis of that reference Cartesian coordinate system.
When the vector sum of the two aforementioned component vectors is determined, a resultant vector is produced, which ideally corresponds with the Earth""s magnetic field vector, both in magnitude and direction. As will be appreciated, the electronic compass performs this vector summing operation to determine the directional heading.
In the ideal case (i.e., in the absence of any interfering magnetic fields), as the compass is rotated full circle (360 degrees) while being subjected to the Earth""s magnetic field, the Earth""s magnetic field vector will trace a circle having its center positioned at the origin of the reference Cartesian coordinate system. In that regard, the magnetic field intensity of the Earth""s magnetic field, which corresponds with the magnitude of the Earth""s magnetic field vector, is uniform. Therefore, as the compass is rotated full circle, the magnitude of the Earth""s magnetic field vector is constant. On the other hand, although the Earth""s magnetic field flux lines maintain constant direction, the orientation of those flux lines relative to the orientation of the magnetic sensors in the electronic compass varies while the compass is rotated. This relative difference is represented by the direction (i.e., angle) of the Earth""s magnetic field vector. Therefore, as the compass is rotated fill circle, the direction of the Earth""s magnetic field vector varies, and more particularly, rotates full circle as well. FIG. 1 illustrates a reference Cartesian coordinate system 20 that is used to represent the ideal electrical signals for each of the two data channels of an electronic compass. Those electrical signals are produced by the two orthogonally disposed magnetic sensors of the compass. The reference Cartesian coordinate system 20 includes a point of origin 22. Extending from origin 22 in opposite directions are the two opposing sides 24, 26 of an abscissa 28 (e.g., x-axis). Likewise, the two opposing sides 30, 32 of an ordinate 34 (e.g., y-axis) extend in opposite directions from origin 22. As shown, the opposing sides 24, 26 of abscissa axis 28 are orthogonally positioned with respect to the opposing sides 30, 32 of ordinate axis 34.
Still referring to FIG. 1, the electrical signal outputs for the orthogonally disposed magnetic sensors are represented by component vectors Vx and Vy, respectively. As shown, component vector Vx is a vector having a magnitude Vx, which corresponds to the magnetic field intensity sensed by one of the orthogonally disposed magnetic sensors. Component vector Vx extends in a direction from origin 22 to a point along one of the sides 24, 26 of abscissa 28. As further shown, component vector Vy is a vector having a magnitude Vy, which corresponds to the magnetic field intensity sensed by the other of the orthogonally disposed magnetic sensors. Component vector Vy extends in a direction from origin 22 to a point along one of the sides 30, 32 of ordinate 34. Component vectors Vx and Vy represent the magnetic field intensities measured by the two orthogonally disposed magnetic sensors, respectively. The two vector components Vx and Vy are added together to form a resultant vector Vm, which defines the total compass output. Resultant vector Vm has a magnitude Vm, which corresponds to the resultant (or total) magnetic field intensity sensed by both channels of the electronic compass. Unlike its vector components (vectors Vx and Vy), resultant vector Vm does not necessarily extend along abscissa 28 or ordinate 34. Rather, resultant vector Vm extends in a direction such that an angle xcex1 is formed between it and the positive extending side 24 of abscissa 28. The angle xcex1 corresponds to the directional heading of the compass with respect to the positioning of its two orthogonally disposed magnetic sensors.
FIG. 1 assumes the ideal case in which there is no nearby interfering magnetic field to cause an inaccurate measurement of the compass heading. Both magnetic sensors included within the compass generate a sinusoidal output when rotated in the Earth""s magnetic field. For this ideal case, if the compass is rotated full circle, the resultant vector Vm plots a circle having a radius Vm and centered at the origin 22 of reference Cartesian coordinate system 20. The ideal case illustrated in FIG. 1 does not commonly occur, however. In particular, disturbing nearby magnetic fields frequently exist. Those disturbing fields distort the outputs of the magnetic sensors. One major cause of error is the magnetic field signature of vehicles. Most vehicles have their own distinct magnetic field signatures. Such magnetic field signatures have an effect on the operation of the electronic compass and cause it to display inaccurate results of directional heading. FIG. 2 represents a real case in which the output of the electronic compass is subjected to the magnetic field signature of a vehicle. As is the case with FIG. 1, FIG. 2 illustrates reference Cartesian coordinate system 20 having origin 22. As the electronic compass is rotated full circle, the distorted magnetic field produces a circular locus of points centered at a point O_Noise(x,y), rather than origin 22. Point O_Noise(x,y) is offset from the origin 22 of Cartesian coordinate system 20 by an offset vector Vo. To facilitate understanding, a translated Cartesian coordinate can be defined as having its origin at point O_Noise(x,y) and its abscissa and ordinate extending parallel to the abscissa and ordinate of Cartesian coordinate system.
As a result of the disturbing magnetic field signature of the vehicle, the compass output vector Vm falsely suggests that the direction of the compass heading is the direction corresponding with the angle xcex2, when it is actually the direction corresponding with the angle xcex1. The actual magnetic field vector Va is obtained by subtracting the offset vector Vo from the compass output vector Vm. Vector Va extends in a direction xcex1, which corresponds with the actual direction of compass heading. The angle xcex1 can be determined by performing familiar trigonometric functions. For instance, xcex1 can be determined by taking the arctangent of the respective component vectors for vector Va.
As is appreciated by those skilled in the art, once the noise center (O_Noise(x,y)) is determined, it is relatively easy to calibrate an electronic compass. Several calibration techniques have been used in the past. The methods of calibration used in the past can generally be classified in two categories. The first such category is manual calibration techniques. The second category is automatic calibration techniques.
The manual calibration techniques generally require that the calibration be performed each and every time the magnetic field signature of the vehicle appreciably changes. As is well known by those skilled in the art, the vehicle load has a substantial effect on the magnetic field signature of the vehicle. As passengers and/or cargo enter and/or exit the vehicle, the magnetic field signature varies. Accordingly, the compass user must calibrate the compass often. As will be appreciated, this task is arduous and time consuming.
The majority of manual calibration techniques also require the user to conduct both a northerly calibration of the compass and a bidirectional calibration of the compass. On top of all of the foregoing, these manual techniques require that the outside environment be free from interfering magnetic fields. The conventional automatic calibration techniques typically adopt a search and match algorithm. In a first example, the calibrating system of the compass stores the magnetic field strength vectors and identifies the occurrence of the vehicle having completed a full turn of one hundred eighty (180) degrees. The calibrating system then determines the correct magnetic strength vector and calibrates the compass to compensate for the interfering magnetic field resulting from the magnetic signature of the vehicle. In a second example of conventional automatic calibration techniques, the calibrating system stores a variety of magnetic field strength vectors, and determines a pair of vertical points and a pair of horizontal points symmetrically positioned with respect to the noise center. In order to determine these points, the vehicle inherently must complete at least a turn of one hundred eighty (180) degrees. From these vertical and horizontal pairs of symmetrical points, the noise center can be obtained and the compass can be calibrated to compensate for the interfering magnetic field resulting from the magnetic signature of the vehicle.
The aforementioned conventional automatic calibration techniques require the outside environment to be free from interfering magnetic fields. Because they are not extremely efficient, these techniques often require the vehicle to be rotated in several circles, which is cumbersome and sometimes impractical to implement. U.S. Pat. No. 5,165,269 (Nguyen), issued Nov. 24, 1992, the disclosure of which is hereby incorporated herein by reference, discloses an electronic compass calibration technique in which the vehicle must be driven in a deliberate circular path. While the vehicle is driven as such, four specific reference points, namely a pair of symmetrical vertical points and a pair of symmetrical horizontal points, are determined. Those reference points represent the orthogonal outputs of the compass as they cross the x-axis and y-axis of a reference Cartesian coordinate system. Those outputs permit the computation of a calibrated compass heading. After the initial calibration, the compass may be calibrated again while the vehicle is moving, but the vehicle must move through a closed loop before each such subsequent calibration.
Similar to the other conventional automatic calibration techniques, the electronic compass calibration technique disclosed in U.S. Pat. No. 5,165,269 requires the user to drive the vehicle in a circular path during the initial calibration process and requires that a pair of vertical points and a pair of horizontal points be determined in order to perform the calibration. Furthermore, the calibration technique disclosed therein does not allow for subsequent calibrations to be made on a continuous basis. Rather, the technique only permits such subsequent calibrations when the vehicle has moved in a closed loop.
U.S. Pat. No. 5,161,811 (Esmer et al.), issued Nov. 10, 1992, the disclosure of which is hereby incorporated herein by reference, discloses an electronic compass calibration technique that, according to its disclosure, does not require user intervention or the need to drive the vehicle in a circular path at the outset. In order to conduct the calibration technique, however, the maximum and minimum voltage values for the two orthogonally disposed sensing windings must be determined. Although the calibration technique disclosed in U.S. Pat. No. 5,161,811 apparently does not require that the vehicle be driven in a deliberate circular path, as a practical matter, the compass cannot be calibrated in accordance with that technique unless the vehicle is driven in a select angular path that is at least semi-circular. In that regard, it will be appreciated by those skilled in the art that the maximum and minimum voltage values for the sensing windings cannot be determined unless the vehicle is driven in at least a semi-circular path. Furthermore, for each subsequent calibration of the compass, the vehicle must be driven in the select angular path to determine the maximum and minimum voltage values for the sensing windings. Accordingly, the calibration technique disclosed in U.S. Pat. No. 5,161,311 does not, as stated therein, permit for continuous adjustment of the compass calibration settings. Rather, the vehicle must be driven in the select angular path each and every time the compass calibration process is performed.
In light of the foregoing, there exists a substantial need in the art for an electronic compass calibration technique that does not require the user to drive the vehicle in a predetermined path in order to conduct the initial calibration of the compass and any necessary subsequent calibrations. Furthermore, there exists a substantial need in the art for an electronic compass calibration technique that can be performed at any time while the vehicle is moving and is not dependent upon driving the vehicle in an angular path of any substantial significance. In other words, there exists a substantial need in the art for an electronic compass calibration technique that is truly automatic.
The present invention is directed to a method and system for continuously and automatically calibrating the settings of an electronic compass, as needed, while a vehicle moves from place to place. The calibration technique of the present invention measures the local magnetic field while the vehicle moves from place to place and determines points on a reference Cartesian coordinate system. Each such point defines the endpoint for a vector corresponding with the measured field intensity and orientation of the local magnetic field. Only any three such points are required. They are used to calculate the center of a circle that extends through all of those points. A vector that begins at the origin of the reference Cartesian coordinate system and terminates at the calculated center of the circle is thereafter subtracted from subsequent field measurement vectors that correspond to the field intensity and direction of the local magnetic field. As a result, the electronic compass compensates for interfering magnetic fields resulting from the varying magnetic field signatures of its associated vehicle.
Accordingly, it is an object of the present invention to provide a new and improved method and system for calibrating the output of an electronic compass.
It is another object of the present invention to provide an electronic compass calibration technique that is truly automatic. It is further object of the present invention to provide an electronic compass calibration technique that does not require the user to drive the vehicle in a predetermined path in order to conduct the initial calibration of the compass. It is still another object of the present invention to provide an electronic compass calibration technique that does not require the user to drive the vehicle in a predetermined path in order to conduct calibrations subsequent to the initial calibration of the compass.
It is yet another object of the present invention to provide an electronic compass calibration technique that can be performed at any time while the vehicle is moving.
It is another object of the present invention to provide an electronic compass calibration technique that is not dependent upon driving the vehicle in an angular path of any substantial significance. It is a further object of the present invention to provide an electronic compass calibration technique that permits proper initial and subsequent calibration upon driving the vehicle in an angular path that is substantially less than one hundred eighty degrees. It is still yet another object of the present invention to provide an electronic compass calibration technique that requires only three measurements of magnetic field intensity and direction.
These and other objects of the preferred form of the invention will become apparent from the following description. It will be understood, however, that an apparatus could still appropriate the claimed invention without accomplishing each and every one of these objects, including those gleaned from the following description. The appended claims, not the objects, define the subject matter of the invention. Any and all objects are derived from the preferred form of the invention, not necessarily the invention in general.